Semiconductor device and method of forming curved image sensor region robust against buckling

ABSTRACT

A semiconductor wafer has a plurality of non-rectangular semiconductor die with an image sensor region. The non-rectangular semiconductor die has a circular, elliptical, and shape with non-linear side edges form factor. The semiconductor wafer is singulated with plasma etching to separate the non-rectangular semiconductor die. A curved surface is formed in the image sensor region of the non-rectangular semiconductor die. The non-rectangular form factor effectively removes a portion of the base substrate material in a peripheral region of the semiconductor die to reduce stress concentration areas and neutralize buckling in the curved surface of the image sensor region. A plurality of openings or perforations can be formed in a peripheral region of a rectangular or non-rectangular semiconductor die to reduce stress concentration areas and neutralize buckling. A second semiconductor die can be formed in an area of the semiconductor wafer between the non-rectangular semiconductor die.

FIELD OF THE INVENTION

The present invention relates in general to semiconductor devices and,more particularly, to a semiconductor die and method of forming a curvedimage sensor region robust against buckling.

BACKGROUND

Semiconductor devices are commonly found in modern electronic products.Semiconductor devices vary in the number and density of electricalcomponents. Semiconductor devices perform a wide range of functions suchas analog and digital signal processing, sensors, transmitting andreceiving electromagnetic signals, controlling electronic devices, powermanagement, and audio/video signal processing. Discrete semiconductordevices generally contain one type of electrical component, e.g., lightemitting diode (LED), small signal transistor, resistor, capacitor,inductor, diodes, rectifiers, thyristors, and powermetal-oxide-semiconductor field-effect transistor (MOSFET). Integratedsemiconductor devices typically contain hundreds to millions ofelectrical components. Examples of integrated semiconductor devicesinclude microcontroller, image sensor, application specific integratedcircuits (ASIC), power conversion, standard logic, amplifier, clockmanagement, memory, interface circuit, and other signal processingcircuits.

An image sensor is a type of semiconductor device that detects andrecords an image by converting light or electromagnetic radiation intoelectric signals. An image sensor can be implemented with semiconductorcharge-coupled devices (CCD) and active pixel sensors in complementarymetal-oxide-semiconductor (CMOS) or N-type metal-oxide-semiconductor(NMOS) technologies with applications in digital cameras, videorecorders, medical imaging equipment, night vision equipment, thermalimaging devices, radar, sonar, and other image detecting devices.

Light from the image scene is typically focused onto a flat or planarimage sensor surface through one or more optical lenses, e.g., up tofour or more lenses. The optical focusing lenses add cost, complexity,and height to the camera package. Even with optical lenses, imagequality is often better in the center region and less on the edges ofthe image sensor. Image sensors are continually driving towards higherresolution, faster focus times, better focus depth, lower profile, andlower cost.

One approach to reducing the number of optical focusing lenses is tomake the image sensor with a curved surface. A camera using a curvedimage sensor is known to have certain performance advantages over onewith a flat image sensor, for example in mobile digital cameraapplications.

FIG. 1 shows semiconductor wafer 10 with base substrate material 12. Aplurality of semiconductor die 14 is formed on wafer 10 separated by aninter-die wafer area or saw street 16. Semiconductor die 14 contains animage sensor region, as described above. Semiconductor die 14 has arectangular or square form-factor with linear side edges 20 and corners22 and a flat surface. Semiconductor wafer 10 is typically reduced inthickness and singulated through saw street 16 into individual thinsemiconductor die 14.

The thin rectangular semiconductor die 14 with a flat surface is placedover a mold or substrate with a curved or concave recess. The surface ofthe thin semiconductor die 14 is deflected by air pressure or otherforces into the concave recess of the mold to form a curved image sensorregion 32, as shown in FIGS. 2a and 2 b.

A rectangular form factor of semiconductor die 14 does not readily fitinto a curved recess. The additional surface area of the rectangular dieform factor over the curved recess creates stress concentration areas.The rectangular image sensor die 14 is subjected to stress when forcedinto the curved recess of the mold. The stress concentration areas cancause out-of-plane deformation or buckling, as shown in area 34 of imagesensor region 32, as shown in FIG. 2a . FIG. 2b shows another example ofbuckling in area 36 of image sensor region 32. The buckling is dependenton die form factor, die size, die thickness, loading, and ratio of diewidth to radius of curvature. A rectangular semiconductor die does notreadily conform to a concave recess. A smaller die with smaller radiusof curvature is more susceptible to buckling. Mechanical instabilitywhere compressive forces exceed stiffness of the base substrate materialcan lead to buckling. In some cases, semiconductor die 14 with imagesensor region 32 is susceptible to sensor cracking and othermanufacturing defects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a known semiconductor wafer with rectangular die;

FIGS. 2a-2b illustrate buckling in a curved image sensor die;

FIGS. 3a-3d illustrate various embodiments of a semiconductor wafer witha plurality of non-rectangular, circular, and elliptical semiconductordie;

FIGS. 4a-4e illustrate the semiconductor die with non-rectangular formfactors singulated from the wafers of FIGS. 3a -3 d;

FIGS. 5a-5c illustrate a process of forming a curved or concave imagesensor region with a mold having a curved surface;

FIGS. 6a-6c illustrate another process of forming the concave imagesensor region with a mold having a curved surface;

FIGS. 7a-7b illustrate the semiconductor die from the wafer of FIGS.3a-3d with a curved or concave image sensor region robust againstbuckling; and

FIGS. 8a-8b illustrate a rectangular semiconductor die with a curvedimage sensor region and openings or perforations in a peripheral regionof the die.

DETAILED DESCRIPTION OF THE DRAWINGS

The following describes one or more embodiments with reference to thefigures, in which like numerals represent the same or similar elements.While the figures are described in terms of the best mode for achievingcertain objectives, the description is intended to cover alternatives,modifications, and equivalents as may be included within the spirit andscope of the disclosure. The term “semiconductor die” as used hereinrefers to both the singular and plural form of the words, andaccordingly, can refer to both a single semiconductor device andmultiple semiconductor devices.

FIG. 3a shows a semiconductor wafer 100 with a base substrate material102, such as silicon, germanium, aluminum phosphide, aluminum arsenide,gallium arsenide, gallium nitride, indium phosphide, silicon carbide, orother bulk semiconductor material. A plurality of semiconductor die 104is formed on wafer 100 separated by inter-die wafer area 106. Inparticular, semiconductor die 104 have a non-rectangular form factor andare arranged in a pattern to maximize the number of die on the wafer andoptimize wafer layout efficiency. For example, semiconductor wafer 100with semiconductor die 104 having a non-rectangular form factor providesa wafer area utilization of about 85%. Each semiconductor die 104 has anactive surface containing an image sensor region 108 implemented as CCDor active pixel sensors in CMOS or NMOS. In one embodiment,semiconductor wafer 100 has a width or diameter of 100-450 millimeters(mm) and thickness of 675-775 micrometers (μm). In another embodiment,semiconductor wafer 100 has a width or diameter of 150-300 mm.

FIG. 3b shows semiconductor wafer 110, with similar materials anddimensions as semiconductor wafer 100, with a base substrate material112. A plurality of semiconductor die 114 is formed on wafer 110. Awafer area 116 is shown between and adjacent to semiconductor die 114for other utilization. In particular, semiconductor die 114 have anon-rectangular form factor and are arranged in a pattern to maximizethe number of die on the wafer and optimize wafer layout efficiency. Forexample, semiconductor wafer 110 with semiconductor die 114 having anon-rectangular form factor and wafer area 116 provides a wafer areautilization of about 99.5%. Each semiconductor die 114 has an activesurface containing an image sensor region 118 implemented as CCD oractive pixel sensors in CMOS or NMOS.

FIG. 3c shows semiconductor wafer 120, with similar materials anddimensions as semiconductor wafer 100, with a base substrate material122. A plurality of semiconductor die 124 is formed on wafer 120separated by an inter-die wafer area 126. In particular, semiconductordie 124 have a circular or elliptical form factor and are arranged in apattern to maximize the number of die on the wafer and optimize waferlayout efficiency. For example, semiconductor wafer 120 withsemiconductor die 124 having a circular form factor provides a waferarea utilization of about 82.7%. Each semiconductor die 124 has anactive surface containing an image sensor region 128 implemented as CCDor active pixel sensors in CMOS or NMOS.

FIG. 3d shows semiconductor wafer 130, with similar materials anddimensions as semiconductor wafer 100, with a base substrate material132. A plurality of semiconductor die 134 is formed on wafer 130separated by an inter-die wafer area 136. In particular, semiconductordie 134 have a circular or elliptical form factor and are arranged in apattern to maximize the number of die on the wafer and optimize waferlayout efficiency. Each semiconductor die 134 has an active surfacecontaining an image sensor region 138 implemented as CCD or active pixelsensors in CMOS or NMOS.

In FIG. 4a , semiconductor wafer 100 is inverted and mounted with activesurface 140 oriented to backgrinding tape 142. Active surface 140contains image sensor region 108. Back surface 144 undergoes abackgrinding operation with grinder or grinding wheel 146 to remove aportion of base substrate material 102 down to surface 148.Semiconductor wafer 100 has a post-grinding thickness of 10-50 μm.Semiconductor wafer 100 is singulated into individual semiconductor die104. Given the non-rectangular form factor of semiconductor die 104 andasymmetric inter-die wafer area 106, a lithographic patterning stepfollowed by a plasma etch or plasma dicing is used to singulatesemiconductor wafer 100 along the side edges of each die. A similarback-grind and singulation is performed on semiconductor wafers 110,120, and 130.

FIG. 4b shows the thin semiconductor die 104 post singulation.Semiconductor die 104 has a non-rectangular form factor, e.g., shapewith non-linear edges 150 on four sides between corners 152 around imagesensor region 108. In particular, non-linear side edges 150 concave intoimage sensor region 108. With the non-rectangular form factor andnon-linear concave side edges 150, there is less base substrate material102 between corners 152 than would be found in the linear side edges 20between corners 22 of the rectangular die 14 of FIG. 1. Semiconductordie 114 singulated from semiconductor wafer 110 has similar features assemiconductor die 104.

FIG. 4c further illustrates the difference between non-rectangularsemiconductor die 104 and rectangular semiconductor die 14. Dotted lines154 illustrate additional base substrate material 155 that would bepresent with linear side edges 20 between corners 22 of the rectangulardie 14 of FIG. 1. The non-rectangular form factor of semiconductor die104 eliminates or removes base substrate material 155 within dottedlines 154 in a peripheral region of semiconductor die 104.

FIG. 4d shows the thin semiconductor die 124 post singulation.Semiconductor die 124 has a circular or elliptical form factor withround side edge 156 around image sensor region 128. With the circular orelliptical form factor and round side edge 156, there is less basesubstrate material 122 than would be found in linear side edges 20between corners 22 of the rectangular die 14 of FIG. 1. Semiconductordie 134 singulated from semiconductor wafer 130 has similar features assemiconductor die 124.

FIG. 4e further illustrates the difference between circularsemiconductor die 124 and rectangular semiconductor die 14. Dotted lines158 illustrate additional base substrate material 159 that would bepresent with linear side edges 20 between corners 22 of the rectangulardie 14 of FIG. 1. The circular form factor of semiconductor die 124eliminates or removes base substrate material 159 within dotted lines158 in a peripheral region of semiconductor die 124.

FIG. 5a shows mold or substrate 160 with curved or concave recess 162 insurface 164. In FIG. 5b , the thin semiconductor die 104 is positionedover mold 160 with surface 148 oriented to recess 162. In FIG. 5c ,surface 148 of semiconductor die 104 is brought into contact withsurface 164 of mold 160. Active surface 140 is deflected by airpressure, hydrostatic pressure, or other forces F, if desired under anelevated temperature, into concave recess 162 to form a curved orconcave image sensor region 108 in semiconductor die 104.

In another embodiment, the thin semiconductor die 104 is positioned overmold 160 with surface 148 oriented to recess 162. An epoxy 170 isdisposed around curved recess 162. In FIG. 6b , surface 148 ofsemiconductor die 104 is brought into contact with epoxy 170 and surface164 of mold 160. In FIG. 6c , active surface 140 is deflected by airpressure, hydrostatic pressure, or other forces F, if desired under anelevated temperature, into concave recess 162 to form a curved orconcave image sensor region 108 in semiconductor die 104. Epoxy 170uniformly disperses to cover surface 148 within concave recess 162. Asimilar process as described in FIGS. 5a-5c and 6a-6c is used to form acurved or concave image sensor regions 118, 128, and 138 insemiconductor die 114, 124, and 134, respectively.

FIG. 7a shows a prospective view of curved image sensor region 108 innon-rectangular semiconductor die 104. In particular, curved imagesensor region 108 exhibits no buckling or at least is robust againstbuckling during or after the mold process of FIGS. 5a-5c and 6a-6c bynature of the non-rectangular form factor with non-linear side edges150. The concave aspect of non-linear side edges 150 into image sensorregion 108 eliminates base substrate material 102 between corners 152,as described in FIG. 4c . There is less base substrate material 102subject to deformation in high stress concentration areas. In otherwords, non-rectangular semiconductor die 104 exhibits a better fit inconcave recess 162 with less base substrate material 102 between corners152. The non-rectangular semiconductor die 104 experiences less stressduring formation of the curved image sensor region 108 in concave recess162. Compressive stress concentration areas on curved image sensorregion 108 are alleviated, which eliminates or reduces buckling duringor after the mold process of FIGS. 5a-5c and 6a-6c . The curved imagesensor region 118 in semiconductor die 114 eliminates or reducesbuckling for the same reasons as described above.

FIG. 7b shows a prospective view of curved image sensor region 128 incircular or elliptical semiconductor die 124. In particular, curvedimage sensor region 108 exhibits no buckling or at least is robustagainst buckling during or after the mold process of FIGS. 5a-5c and6a-6c by nature of the circular or elliptical form factor with roundside edge 156. The round side edge 156 eliminates base substratematerial 122, as described in FIG. 4e . There is less base substratematerial 122 subject to deformation in high stress concentration areas.In other words, circular or elliptical semiconductor die 124 exhibits abetter fit in concave recess 162. The circular or ellipticalsemiconductor die 104 experiences less stress during formation of thecurved image sensor region 108 in concave recess 162. Compressive stressconcentration areas on curved image sensor region 108 are alleviated,which eliminates or reduces buckling during or after the mold process ofFIGS. 5a-5c and 6a-6c . The curved image sensor region 138 insemiconductor die 134 eliminates or reduces buckling for the samereasons as described above.

In another embodiment, FIG. 8a shows a prospective view of curved imagesensor region 180 in rectangular semiconductor die 182. To reduce oreliminate buckling, a plurality of openings 184 is formed in high stressconcentration areas, i.e., areas of greatest compressive stressescommonly subject to buckling, such as proximate to the outer side edgeof image sensor region 180 or a peripheral region of semiconductor die182. Openings 184 remove a portion of the base substrate material in aperipheral region of semiconductor die 182 to reduce compressive stressconcentration areas on curved image sensor region 180. The amount ofbase substrate material removed by openings 180, as well as the shapeand location of the openings, depend on base substrate material geometryand degree of bending. Openings 184 can also be formed in anon-rectangular semiconductor die e.g., 104, 114 to further reducecompressive stress concentration areas on curved image sensor region108, 118.

FIG. 8b shows a prospective view of curved image sensor region 190 inrectangular semiconductor die 192. To reduce or eliminate buckling, oneor more perforation or cut-out 194 are formed in high stressconcentration areas, i.e., areas of greatest compressive stressescommonly subject to buckling, such as proximate to the outer side edgeof image sensor region 190 or in a peripheral region of semiconductordie 192. Perforation 194 remove a portion of the base substrate materialin a peripheral region of semiconductor die 192 to reduce compressivestress concentration areas on curved image sensor region 190. The amountof base substrate material removed by perforation 194, as well as theshape and location of the perforation, depend on base substrate materialgeometry and degree of bending. Perforation 194 can also be formed in anon-rectangular semiconductor die e.g., rectangular or non-rectangularsemiconductor die 104, 114 to further reduce compressive stressconcentration areas on curved image sensor region 108, 118.

The non-rectangular semiconductor die 104 and 114 and circular orelliptical semiconductor die 124 and 134 provide form factors thatremove base substrate material, found in rectangular semiconductor die,and avoid stress concentration areas. Likewise, openings 184 andperforations 194 reduce stress concentration areas that leads tobuckling, warpage or cracking of the semiconductor die. Reducing stressconcentration areas in image sensor regions 108, 118, 128, and 138eliminates or reduces buckling, cracking, and instability during themanufacturing process and provides a more reliable curved image sensorsemiconductor die.

The areas of removed base substrate material can be used a variety forof purposes, e.g., wirebond, additional circuitry, and additionstructural elements for packaging, such as adhesives or standoffmaterial. Non-active die areas can be used for alignment, calibrationstructures, or for additional die types. In particular, wafer area 116in FIG. 3b or wafer area 126 in FIG. 3c can be used to form smallersemiconductor die of the same type or a different type as semiconductordie 114 or 124, thus increase usage of the semiconductor wafer area. Forexample, semiconductor die 114 can be a large image sensor die andsemiconductor wafer area 116 can be a small image sensor die.

While one or more embodiments have been illustrated and described indetail, the skilled artisan will appreciate that modifications andadaptations to those embodiments may be made without departing from thescope of the present disclosure.

What is claimed:
 1. A method of making a semiconductor device,comprising: providing a semiconductor wafer including a base substratematerial; singulating the semiconductor wafer into a plurality ofnon-rectangular semiconductor dies by removing stress concentrationareas in the base substrate material along each side edge of thenon-rectangular semiconductor dies leaving symmetrical, non-linear sideedges extending between each corner of the non-rectangular semiconductordies; forming a plurality of openings or perforations along each sideedge of the non-rectangular semiconductor dies; and forming a curvedsurface in the non-rectangular semiconductor dies including along eachnon-linear side edge without the stress concentration areas.
 2. Themethod of claim 1, further including forming an image sensor region inthe non-rectangular semiconductor dies.
 3. The method of claim 1,further including forming a second semiconductor die in an area of thesemiconductor wafer between the non-rectangular semiconductor dies. 4.The method of claim 1, further including singulating the semiconductorwafer with a plasma process.
 5. A method of making a semiconductordevice, comprising: providing a semiconductor wafer including a basesubstrate material and a plurality of semiconductor dies formed in thebase substrate material; removing a portion of the base substratematerial in stress concentration areas of the base substrate materialalong each side edge leaving non-linear side edges extending betweencorners of the semiconductor dies; and forming a curved surface in thesemiconductor dies including along each non-linear side edge.
 6. Themethod of claim 5, wherein removing the portion of the base substratematerial includes forming the semiconductor dies with a non-rectangularform factor.
 7. The method of claim 5, wherein removing the portion ofthe base substrate material includes: forming the semiconductor dieswith a rectangular form factor; and forming a plurality of openings orperforations in the semiconductor dies.
 8. The method of claim 5,wherein removing the portion of the base substrate material includesforming a plurality of openings or perforations along the side edges ofthe semiconductor dies.
 9. The method of claim 5, further includingforming an image sensor region in the semiconductor dies.
 10. The methodof claim 5, further including singulating the semiconductor wafer toseparate the semiconductor dies.
 11. The method of claim 10, furtherincluding singulating the semiconductor wafer with a plasma process. 12.A method of making a semiconductor device, comprising: providing asemiconductor wafer including a base substrate material and a pluralityof semiconductor dies formed in the base substrate material; singulatingthe semiconductor wafer to provide the semiconductor dies devoid of aportion of the base substrate material in stress concentration areasalong each side edge of the semiconductor dies; and forming a curvedsurface in the semiconductor dies including along each side edge. 13.The method of claim 12, further including forming the semiconductor dieswith a non-rectangular form factor.
 14. The method of claim 13, whereinthe non-rectangular form factor includes non-linear edges.
 15. Themethod of claim 12, further including forming an image sensor region inthe semiconductor dies.
 16. The method of claim 12, further includingforming a plurality of openings or perforations along the side edges ofthe semiconductor dies.